The present invention relates to a process of forming a permeable gas separation material.
Poly(phenylene oxide) (PPO) is known as a moderately permeable polymer membrane material having alternating aromatic cycles and C-O linkages in the main chain. Among the many aromatic polymers having a high glass transition temperature, PPO shows the highest permeability to gases, higher than polysulfone or bisphenyl-A-polycarbonate which have rather similar structures of repeat units. Whatever the reason for this, PPO has received significant attention as a permselective material.
However, measurements of separation factors for PPO have not been fully consistent, possibly because PPO can crystallise under some conditions and variations in crystallinity can cause variations in permeability.
It has been proposed to increase the permeability of PPO by the introduction of bulky substituents.
Thus, European patent application EP 0 360 318 (Eniricerche SpA) describes a modified poly (2,6-dimethyl-p-oxyphenylene) with a glass transition temperature in the range 180xc2x0 to 220xc2x0 C. containing a hydroxy butyl group replacing the methyl group in the 2-position, or a hydroxyethyl group in the 3-position.
United States patent U.S. Pat. No. 5,169,416 (Pedretti et al. / Snam SpA et al.) describes a process of modifying PPO by introducing a trialkylsilyl halide substituent group.
United States patent U.S. Pat. No. 4,586,939 (Li / The Standard Oil Company) describes PPO which is substituted in the 3-and 5-positions with a radical larger than chloride, such as bromide.
Gas separation membranes prepared from PPO of low molecular weight have a tendency to rupture.
An important property of a gas separation membrane is its permselectivity, especially if one or more components of the gas mixture comprise polar molecules. In fact, for such mixtures, permselectivity is more important than permeability.
It is an object of the present invention to provide a permeable gas separation material having good mechanical strength and good permselectivity.
We have discovered that this object, and other useful benefits, can be achieved by sulphonating a partially brominated PPO.
Thus, according to the invention there is provided a process of forming a permeable gas separation material characterised by sulfonating a partially brominated poly(phenylene oxide).
The partially brominated PPO, which is used as a starting material for the sulfonation process, preferably is from 20% to 60% brominated PPO.
The partially brominated PPO is preferably partially brominated poly (2,6dimethyl-1,4-phenylene oxide), although the invention is equally applicable to partially brominated poly (phenylene oxide)s after other structures, for example where the substituents at the 2- and 6- positions are selected independently from C1 to C8 aliphatic, C5 to C7 cycloaliphatic, C1 to C8 alkoxy; C6 to C12 aromatic radicals or inert substituted derivatives thereof.
The process may comprise reacting the partially brominated PPO with a sulfonating agent in a non-polar solvent, for example using a procedure similar to the known procedure for sulfonating low MW PPO. The sulfonating agent is typically chlorosulfonic acid. The non-polar solvent may be chloroform. The concentration of the PPO in the non-polar solvent is preferably less than 10% by weight. Sufficient sulfonating agent may be used to nominally sulfonate un-reacted repeat units in the partially brominated PPO.
The gas separation material formed by the process according to the invention may be made into a membrane. A membrane may be formed by coating a flat surface or the lumen or the shell side of hollow fibres with dilute solutions of the modified polymer in a suitable solvent and allowing the solvent to evaporate.
The invention also provides a method of separating component gases from a gas mixture, comprising contacting the gas mixture with a gas separation material formed by sulfonating a partially brominated PPO.
The preferred gas separation material as formed by sulfonating a partially brominated PPO, has a CO2/CH4 permeability ratio of at least 51.0 for pure gases and/or a CO2/CH4 separation factor of at least 22, measured with a CO2/ CH4 mixture containing 19.3% CO2.